The invention generally relates to systems and methods for fatigue or stress testing to fracture one or more mesh grid tubes such as implantable medical stents. More specifically, the invention relates to systems and methods for stressing a stent or mesh tube to fracture or break which provides valuable data for estimating the usable life of such devices.
Modern medical procedures routinely include the employment of implants into a patient's vascular system to perform various therapeutic functions. Prosthetic vascular implants, such as heart-valves, stents, grafts, mesh tubes, and stent-grafts used for human implantation are subjected to the continuous fluctuating stress of blood pressure. As an example, tubular mesh samples or stents are often inserted in an artery of a patient to maintain a flow lumen through the artery at a location that had previously been at least partially blocked or occluded. It is therefore necessary to test such implants to prove their durability over a lifetime of exposure to pulsatile blood pressure. Ideally such stents, mesh samples, or other vascular prostheses, are able to withstand the physiological dynamics that occur within the vessel or organ in which they are emplaced. For instance, in the abdominal aorta, blood pressure in the average healthy subject is 120 mm Hg/80 mm Hg, i.e. the blood pressure varies by 40 mm Hg for every pulse. Compliance of a healthy aorta can be of the order of 20-25% per 100 mm Hg so that a change in vasculature diameter of 8 to 10% can be expected at every heartbeat. In order to simulate such a change in diameter, testers employ a pulse pressure between 80 mm Hg and 100 mm Hg. Typically, in testing to success, vascular implants are tested for 400,000,000 cycles which represent approximately 10 years of implantation life at a heart rate of 80 beats per minute.
Testing to success is indicative of the durability of the stent under physiological conditions of systolic/diastolic pressures encountered in accelerated radial pulsatile durability testing. However, testing to success does not predict the endurance limit or fatigue life of the stent, i.e., there is no way to know under what conditions, including conditions that may exceed physiological parameters, the stent or stent graft would fail.
To address this weakness, new regulations (FDA, ASTM, ISO) are being developed that outline test requirements that are now concerned with predicting fatigue lifetime of the stent or stent graft and require stent manufacturers to test their products under a ‘testing to failure’ or ‘test to fracture’ regime, so that stents and stent grafts may be tested up to their endurance limit. An alternative method that is being rapidly pursued and evaluated is a “Fatigue to Fracture” approach. A rudimentary technique that is more akin to aerospace testing, this methodology involves a combination of Finite Element Analysis (FEA) modeling and in vitro testing to assess the durability of stents through established fracture mechanics techniques. These testing guidelines and standards are still under development, i.e. ASTM F04.30.06.
Knowing when and where fracture, secondary fracture, or other failure, of the stent, mesh tubes, or other prosthesis, is most likely to occur under a variety of simulations is ideal to device development. Manufacturers can then use this information to redesign their product with the knowledge gained by fatigue to fracture analysis. Providing a stent, or other prosthesis, of suitable strength and durability for lasting implantation into a patient, to minimize the likelihood of failure is desirable. Determining the approximate fatigue and endurance location limits of the stent, or other prosthesis, helps accomplish the provision of a suitable stent, or other prosthesis.